Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each semiconductor die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual semiconductor die from the finished wafer and packaging the die to provide structural support and environmental isolation. The term “semiconductor die” as used herein refers to both the singular and plural form of the words, and accordingly, can refer to both a single semiconductor device and multiple semiconductor devices.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller semiconductor die size can be achieved by improvements in the front-end process resulting in semiconductor die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
A semiconductor die typically contains an interconnect structure for mounting the die to a substrate. For example, the interconnect structure can be a bump or conductive pillar formed over contact pads within an opening in an insulating layer on the semiconductor die. The bump or conductive pillar is bonded to the substrate by reflowing the bump material to provide mechanical and electrical interconnect between the semiconductor die and substrate. Conductive pillars offer the advantage of smaller interconnect pitches and higher interconnect and routing density.
A seed layer is usually necessary between the conductive pillar and contact pad of the semiconductor die for good adhesion. The seed layer is deposited over the insulating layer and contact pads of the semiconductor die prior to forming the conductive pillars, and then removed from areas outside a footprint the conductive pillar, often by a wet etching process. The wet etch is known to remove a portion of the seed layer under the conductive pillars, i.e., the wet etch undercuts the seed layer beneath the conductive pillars. However, undercutting the seed layer beneath the conductive pillars weakens the adhesion between the conductive pillars and contact pads of the semiconductor die, leading to joint cracking and manufacturing reliability problems. The smaller interconnect pitch increases the occurrence of seed layer undercutting due to an inability to precisely control the wet etch rate. In addition, the conductive pillars are formed up to an edge of the insulating layer over the semiconductor die. A high current density exists around the base of the conductive pillars adjacent to the insulating layer over the semiconductor die, which increases interconnect resistance.